Double-sheath pipe for transporting fluids, provided with a device for limiting propagation of a buckle of the outer tube and method for limiting propagation

ABSTRACT

The invention concerns a double-sheath rigid pipe for transporting hydrocarbons consisting of two respectively inner ( 2 ) and outer ( 3 ) coaxial tubes separated by an annular gap ( 5 ), said pipe being adapted to be wound and comprising a device limiting the propagation of a deformation occurring on the outer tube, wherein a radial stop device ( 10 ) linked to the inner tube ( 2 ) but not linked to the outer tube ( 3 ) allows a localized collapse of value G of the outer tube, G being selected to stop the propagation of a deformation of the outer tube ( 3 ).

BACKGROUND OF THE INVENTION

The present invention relates to a device and to a method for limitingor arresting the propagation of a buckle in a double-walled (ordouble-sheath) pipe consisting of two, respectively inner and outer,coaxial tubes separated by an annular space, this pipe being a rigidpipe for transporting fluids such as hydrocarbons, and designed to besubmerged.

A rigid pipe or tube is laid on the seabed usually from a vessel calleda pipelaying barge. The laying is called S-laying when the pipe adoptsthe shape of an S between the pipelaying barge and the seabed and it iscalled J-laying when the pipe adopts the shape of a J. In the lattercase, a guide ramp is provided on the pipelaying barge, which ramp maysometimes be partially immersed in the water.

The rigid pipe to be laid is stored on the pipelaying barge either inpipe sections of a given but relatively short length, the pipe sectionsbeing joined together as the laying progresses, or it is made as a verylong pipe on land and then wound onto a storage reel located on thepipelaying barge, the pipe then being unwound from the said reel duringthe laying operation. These laying operations are described in the API(American Petroleum Institute) document “Recommended Practice 17 A” from1987.

When the pipe has left the barge and while the said pipe is being laid,it is important for the latter not to undergo plastic deformation inbending, which would result in ovalization of the pipe, the saidovalization causing a “weak singularity” which would be conducive to theinitiation of a collapse. Moreover, when the pipe is laid on the seabedat great water depths (typically greater than 300 m and possibly down to2000 m and more), the hydrostatic pressure exerted on the pipe may besufficient to initiate a radial buckle which has a tendency to propagatealong the pipe, in both directions. Of course, the buckle will formpreferentially at a “weak singularity” when one exists on the pipe. Whenthe buckle occurs, it is then necessary to replace at least that sectionor portion of the pipe comprising the buckled or collapsed region. Thebuckle propagation pressure is given by the formula:A×σ₀×(T/D)^(β)where σ₀ is the yield stress of the steel, T is the thickness of thepipe and D is the external diameter of the pipe and where the parametersA and β are given by the American recommendations API RP 1111 (namely,A=24, β=2.4) or Norwegian recommendations DNV OS/F100 (namely, A=26.7,β=2.5). To resist the propagation of a buckle, the correspondingpressure must be greater than the hydrostatic pressure.

To avoid this problem, it is possible to increase the thickness of thepipe so as to have a propagation pressure greater than the hydrostaticpressure, but this results in a considerable increase in both the costand the weight of the pipe, most particularly in the case ofdouble-walled rigid pipes.

To prevent the propagation of a local buckle or local buckles, it hasalso been proposed to provide the pipe with certain devices or means,called buckle arrestors. The API Recommended Practice 1111 gives variousrecommendations and formulae which indicate above which depth thearrestors are recommended, necessary or strictly indispensable. Suchdevices were firstly proposed within the context of single-walled rigidpipes.

The arrestor may be movable, as described in the document U.S. Pat. No.3,747,356. According to that document it is proposed to link a cylinderto a cable, to lodge the cylinder inside a pipe section and then tosimultaneously unreel the pipe and the cable so as to keep the cylinderin the pipe section while the latter is being laid, until the pipe comesinto contact with the seabed. The cylinder is then brought back up so asto be lodged in another pipe section to be laid, which is joined to theprevious section. Consequently, any buckle likely to occur, when layingthe pipe, between the pipelaying barge and the seabed is immediatelyarrested and is therefore not allowed to propagate along the pipesections. However, such an arrangement provides no solution oreffectiveness for arresting buckles likely to be propagated after thepipe has been finally laid on the seabed.

This is why fixed arrestors have been proposed, these being positionedin places along the pipe. The distance separating two arrestors isdefined according to the cost of these devices compared with the cost ofreplacing the crushed section between two devices. In general, thesedevices are positioned along the rigid pipe every 100 to 300 m. Thesedevices, by locally increasing the inertia of the pipe, make it possibleto arrest the propagation of a buckle.

According to a first solution, an inner, or preferably outer,reinforcing collar (possibly in two parts, constituting a “clamp”) isused. Thus, in U.S. Pat. No. 3,768,269, it is proposed that thestiffness of the pipe be increased locally by placing, at regularintervals, reinforcing collars whose length ranges between 1 m and 2.5m. Such a solution is valid only for pipes laid in sections since thereinforcing collars can be mounted and fastened to the pipe sections inthe factory and then transported by the pipelaying barge to the layingsite. When the pipe is long and wound onto a storage reel, it thenbecomes virtually impossible to wind the pipe with its reinforcingcollars onto a reel since they would result in straight or almoststraight portions that cannot be deformed when winding the pipe onto thestorage reel. To mitigate this difficulty, it is conceivable to mountand fasten the reinforcing collars during the laying operations.However, it would then be necessary to interrupt the laying, at regularintervals, so as to mount and fasten the reinforcing collars. Accordingto an alternative solution known through this same patent or throughdocuments GB 1 383 527 or U.S. Pat. No. 5,458,441, the localizedreinforcement may take the form of a thicker intermediate sleeve weldedto the ends of the pipe.

According to a second solution, a spiralled rod is used on the externalwall of the pipe. Thus, to allow the pipe to be wound onto a reel, U.S.Pat. No. 4,364,692 proposes to wind a rod tightly around the pipe so asto form a certain number of turns which can be welded at their ends tothe rod itself and/or to the pipe.

According to another embodiment, the turns may be individual turns, bywelding their two ends and by regularly spacing them apart along thatportion of the pipe to be reinforced.

As long as the pipe is a single-walled pipe, the increase in thediameter in the reinforced portions may be acceptable. However, when thepipe is of the double-walled or pipe-in-pipe type, that is to say onecomprising an outer tube or carrier pipe into which the inner tube orflowline is inserted, the increase in the diameter of the outer tube isunacceptable when transporting and storing long lengths of double-walledpipe when the pipe is laid by the “unreeled rigid pipe” method.

In addition, when the rigid pipe to be laid is manufactured in longlengths on land and then wound onto a reel on the pipelaying barge, thesolutions recommended in the aforementioned documents are notappropriate as they use either long reinforcing collars, having a lengthof about 1 to 2.5 m, as in U.S. Pat. No. 3,768,209, or the winding of areinforcing rod around the rigid pipe, as in U.S. Pat. No. 4,364,692.

In the case of double-walled pipes, the arrestor device generallyconsists of a collar mounted on the outer tube of the pipe. When thepipe is laid using a barge for laying an unreeled rigid pipe, thisdevice must be mounted progressively with the laying operation (it isnot reelable), thereby considerably increasing the cost of laying.

For the purpose of solving these problems and of obtaining double-walledpipes that can be wound, despite the propagation arrestors, theApplicant has already proposed particular devices.

According to Application FR 99/08540, a portion of flexible pipe orspring is welded to the internal wall of the outer tube in order locallyto increase the inertia of the outer tube and form a flexiblepropagation arrestor which is reelable.

According to Application FR 99/15216, the propagation arrestor consistsof an annular compartment filled with resin which is injected before orduring the laying and which can be cured only after the laying if thelength of the compartment for the resin is too great to allow winding inthe cured state. The compartment serves to transfer the loads on theouter tube to the inner tube.

According to Application FR 00/00849, the arrestor consists of a steelpartition which, as in the previous solution, transfers the loads fromthe outer tube to the inner tube.

All these buckle propagation arrestor devices therefore consist inreinforcing the outer tube, which may be subjected to this propagation,thereby directly or indirectly increasing the inertia of the outer tube.

The devices developed by the Applicant give very good results and areeffective down to depths of the order of 2500 m. In some applications,when the depth is smaller, these devices may represent an excessive costcompared with the actual requirements.

SUMMARY OF THE INVENTION

It is an objective of the invention to provide a propagation arrestordevice for a reelable double-walled pipe which is very simple tomanufacture and to mount, and is effective even at great water depths.

The objective of the invention is achieved by means of a method oflimiting the propagation of a buckle appearing on an outer tube of adouble-walled rigid pipe for transporting hydrocarbons, which consistsof two coaxial tubes, the inner tube and the outer tube respectively,separated by an annular space, this pipe being of the reelable type,characterized in that the maximum deformation that the outer tube mayhave, without the said buckle propagating due to the effect of thehydrostatic pressure, is determined and a device linked to the innertube and not linked to the outer tube is inserted into the annular spacebetween the two tubes so as to limit the deformation of the outer tubeto below this maximum deformation. Thus, the invention runs counter tothe known devices; instead of reinforcing the outer tube so as toprevent it from deforming, it is not reinforced but the deformation thatit can withstand is given a maximum value, this maximum value beingdetermined by calculation or by experiment, depending on the hydrostaticpressure and on the characteristics of the outer tube.

The invention also relates to the pipe employing the method of theinvention, namely a double-walled rigid pipe for transportinghydrocarbons, consisting of two coaxial tubes, the inner tube and theouter tube respectively, separated by an annular space, this pipe beingof the reelable type and including a device for limiting the propagationof a buckle appearing on the outer tube, characterized in that the saiddevice consists of a radial stop device linked to the inner tube,allowing a localized collapse of value G of the outer tube, G beingchosen to arrest the propagation of a buckle of the outer tube.

In practice, G is determined in conjunction with the length L of thedevice. This is because the propagation arrestor device forces thedeformation of the outer tube (its ovalization) to “reform” to a largerdiameter, and therefore to go from a relatively elongate ellipticalcross section to a less elongate elliptical cross section with the sameperimeter, this cross section corresponding to a buckle propagationpressure greater than the hydrostatic pressure. However, to arrest thepropagation of the buckle, the device must be long enough to dissipatethe propagation energy. This length must in particular be long enoughfor the “reforming” of the flattened ellipse in the arrestor device notto induce, by a simple geometry effect, a new deformation into aflattened ellipse on the other side of the device, lying at right angleswith respect to the first ellipse, which new deformation could againpropagate.

Although the values of G and L to be used therefore depend entirely onthe case in point, namely the geometrical and physical characteristicsof the double-walled pipe and the ambient hydrostatic pressure for whichit is designed, an empirical rule is to provide a device with a lengthof between 0.3 and 5 times, preferably between 0.5 and 3 times, theoutside diameter of the outer tube and a radial thickness of between 0.1and 0.9 times, preferably between 0.3 and 0.8 times, the thickness ofthe annular space. However, it will be understood that the longer thelength, the smaller the thickness may be, and vice versa.

The invention applies especially to double pipes provided with spacers.The spacers are the spacing rings which centre, uniformly along thepipe, the inner tube coated with the layer of thermal insulation so asto prevent the said thermal insulation coming in contact with the outertube. These spacers are usually placed every 2.5 m. Their height isgenerally about 70% to 80% of the thickness of the annular space andtheir length is less than 100 mm in order to limit heat losses. They aregenerally made of plastic, for example nylon. Although the thickness ofthe spacers may in some cases be compatible with a value correspondingto propagation arrest, in practice they are not used for this purposebecause, on the one hand, their length is too short to dissipate thepropagation energy and, on the other hand, they tend to creep over timeand under load.

In one embodiment, the said device is a continuous or discontinuoussection of the inner tube with an increased thickness.

In another embodiment, the said device is a spring fastened to the innertube.

In another embodiment, the said device consists of a succession of ringsfixed to the inner tube.

In another embodiment, the said device consists of a sleeve placed overthe thermal insulation between two spacers.

In another embodiment, the said device consists of collars placedloosely around the layer of thermal insulation.

In another embodiment, the said device consists of studs held togetherby flexible bands.

In all the embodiments, the device is made of a strong material notliable to creep, for example epoxy resin or steel.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features will become more apparent on reading thefollowing description of the invention, with reference to the appendeddrawings in which:

FIG. 1 is a longitudinal sectional view of part of a double-walled rigidsection equipped with a first embodiment of the propagation arrestordevice according to the invention;

FIG. 2 is a longitudinal sectional view of part of a double-walled rigidsection equipped with a second embodiment of the propagation arrestordevice according to the invention;

FIG. 3 is a longitudinal sectional view of part of a double-walled rigidsection equipped with a third embodiment of the propagation arrestordevice according to the invention;

FIG. 4 is a longitudinal sectional view of part of a double-walled rigidsection equipped with a fourth embodiment of the propagation arrestordevice according to the invention;

FIG. 5 is a longitudinal sectional view of part of a double-walled rigidsection equipped with a fifth embodiment of the propagation arrestordevice according to the invention;

FIG. 6 is a longitudinal sectional view of part of a double-walled rigidsection equipped with a sixth embodiment of the propagation arrestordevice according to the invention;

FIG. 7 is a longitudinal sectional view of part of a double-walled rigidsection equipped with a seventh embodiment of the propagation arrestordevice according to the invention;

FIG. 8 is a cross-sectional view on VIII—VIII of the device in FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The double-walled rigid pipe 1 of longitudinal axis A, shown partiallyin FIG. 1, comprises an inner wall or tube 2 (the “flowline”), thediameter and the nature of the material of which are chosen according tothe fluid flowing in the said inner tube, especially according to thetemperature and pressure of the said fluid, and an outer wall or tube 3(the “carrier pipe”) which is slipped over the inner tube 2. The outertube 3 has an outside diameter which is oversized with respect to theinner tube 2 in order to allow a thermal insulation to be placed in theannular space 5 and has a thickness which makes it possible to withstandthe hydrostatic pressure which is exerted on the said outer tube 3.

According to the invention, the inner tube 2 comprises a cylindricalsection 10 of length L with a thickness increased towards the outside byan amount H, leaving a distance G to the outer tube. G is, bycalculation or by experiment, greater than the value above which abuckle of the outer tube 3 propagates under the hydrostatic pressure,and the length L is sufficient to arrest the propagation of a buckle.

Calculations were made on a pipe structure whose inner tube measured 10inches (273.1 mm) for a thickness of 12.7 mm and whose inner tubemeasured 6 inches (168.3 mm) for a thickness of 18.3 mm, with an annularspace 40 mm in thickness. The buckle propagation pressure without adevice was 11 MPa (110 bar, corresponding to a water depth ofapproximately 1100 m). When the device has a thickness of 20 mm (0.5times the thickness of the annular space), the propagation pressure is16.5 MPa (165 bar, i.e. approximately 1600 m). When the device has athickness of 32 mm (0.8 times the thickness of the annular space), thepropagation pressure is 26 Mpa (260 bar, i.e. approximately 2500 m).

A test was carried out with a double-walled pipe identical to that forthe calculations, forming a propagation arrestor device having athickness of 32 mm and a length of 100 mm. A buckle was initiated on theouter tube under an external pressure of 150 bar (corresponding to thehydrostatic pressure existing at a depth of 1500 m): the buckle wasproperly arrested by the device. It will be understood that if thedevice had had a thickness of 20 mm, the length of this device wouldhave had to be greater so as to dissipate the, propagation energy (thepropagation pressure in this case being higher, but close to theexternal pressure). In contrast, if the device had a smaller thickness,(close to the outside diameter of the inner tube), the buckle would nothave been arrested, the propagation pressure in this case being lessthan the external pressure.

Other illustrative examples of the arrestor devices will now bedescribed.

FIG. 2 shows an arrestor device 11 consisting of a spring wound with atight pitch around the inner tube and welded to it or prevented frommoving translationally by one or two stops, such as the welded ring 6.Using a spring of this type does not increase the rigidity of the innertube.

In FIG. 3, the arrestor device 12 consists of a succession of steelrings which are fixed to the inner tube and have a width decreasing fromthe centre of the device out towards its ends, so as to reduce thestresses on the inner tube during reeling. Alternatively, the rings maybe made of epoxy resin so as to limit the thermal losses at the device.The rings could also be of the same length.

In FIG. 4, which shows the insulation 4 and the spacers 7, the arrestordevice consists of a sleeve 13 placed between two spacers 7 over thethermal insulation 4 and fixed to the spacers. This device isparticularly beneficial as it does not interfere with the thermalinsulation. If necessary, the space separating the spacers may betailored to the length L of the arrestor device.

In FIG. 5, the device 14 is similar to the device in FIG. 1, except thatthe additional thickness is discontinuous (in cross section) instead ofbeing continuous, and it may be made in the form of a succession ofrings or in the form of a helix. Each ring or helix may be formed from asuccession of discrete additional thicknesses.

In FIG. 6, the device 15 consists of two steel collars placed looselyaround the layer of thermal insulation 4.

In FIGS. 7 and 8, the device 16 consists of several circles of studscircumferentially connected together by a flexible band 8. The thermalinsulation 4 is interposed between these circles.

1. A plural tube rigid pipe for fluid transmission wherein the pipebeing intended to be immersed and subjected to hydrostatic pressure, thepipe including coaxial inner and outer tubes, with the tubes beingshaped and positioned to define an annular space between the inner andouter tubes, the inner tube having an inner hollow space to allow fluidto flow therethrough; the rigid pipe and the tubes thereof are of typesto enable the pipe to be reelable; a buckle propagation limiting devicefor limiting propagation of a buckle occurring in the outer tube, thedevice comprising a radial stop device linked to the inner tube and notlinked to the outer tube, wherein the radial stop device is shaped andpositioned as to permit a localized collapse of the outer tube, but thevalue of the permitted localized collapse is chosen to arrest thepropagation of a buckle of the outer tube, and the buckle propagationlimiting device is of a type to enable the pipe to be reelable.
 2. Thepipe of claim 1, wherein the pipe is of a material for transportinghydrocarbons.
 3. The device of claim 1, wherein the device has a lengthalong the pipe of between 0.3 and 5 times the diameter of the outertube, and the device has a thickness in the radial direction of between0.1 and 0.9 times the thickness in the radial direction of the annularspace.
 4. The pipe of claim 1, wherein the device has a length ofbetween 0.5 and 3 times the diameter of the outer tube.
 5. The pipe ofclaim 1, wherein the device has a thickness of between 0.1 and 0.8 timesthe thickness of the annular space.
 6. The pipe of claim 1, wherein thedevice is a continuous section of the inner tube and having an increasedthickness with respect to the thickness of the inner tube.
 7. The pipeof claim 1, wherein the device is comprised of a strong material notliable to creep.
 8. The pipe of claim 7, wherein the device is comprisedof an epoxy resin.
 9. The pipe of claim 7, wherein the device iscomprised of steel.
 10. The pipe of claim 1, wherein the device is adiscontinuous section of the inner tube and having an increasedthickness with respect to the thickness of the inner tube.
 11. The pipeof claim 1, wherein the device comprises a spring fastened to the innertube.
 12. The pipe of claim 1, wherein the device comprises a successionof rings around and fixed to the inner tube.
 13. The pipe of claim 1,further comprising spacers disposed at spaced locations along the pipein the annular space; thermal insulation around the inner tube andbetween the spacers; the device comprising a sleeve placed over thethermal insulation and between neighboring ones of the spacers along thepipe.
 14. The pipe of claim 1, further comprising a layer of thermalinsulation around the inner tube; the device comprising a plurality ofcollars loosely placed around the layer of thermal insulation.
 15. Thepipe of claim 1, wherein the device comprises studs in the annular spaceand flexible bands holding the studs together.